This invention relates to a printing press using a gravure printing process in which a printing member or plate is engraved with an ink receiving matrix, the depth of the engraved matrix determining the quantity of ink which is placed on the substrate and hence the depth of colour applied to the substrate. In a typical gravure printing press, the printing member has the form of a cylindrical roll with the image engraved in its outer surface. The engraved roll is rotated about its longitudinal axis and dips into an ink-containing bath which coats the roll with ink. Ink is scraped from the surface of the engraved roll by a doctor blade which removes all of the ink from the surface and just leaves the ink which remains in the engraved ink receiving matrix. Further rotation of the ink filled roll brings it into contact with the substrate to be printed, typically paper, and the ink is transferred from the matrix onto the surface of the substrate. In colour printing by the gravure process there are two or more such presses arranged in series one downstream of the other and an ink drying station is provided downstream from each press to ensure that the ink on the substrate from that press is dry before the substrate reaches the next press. Such presses are fed by a continuous web of paper and the various printing presses are kept in synchronism with one another so that the impressions from successive printing machines are registered one on top of the other.
Typically, for full colour printing, there are four separate printing presses downstream from one another. Each of the presses provides a printing impression of different colour and in a four colour process the different presses use black, yellow, cyan and magenta coloured inks. Such printing presses are very large and a considerable effort is involved in the preparation of their printing members and in their initial setting up to ensure that the impressions printed by each press are in exact registration with one another and to ensure that they continue to run in synchronism with one another.
Nowadays, the image which is to be printed is usually scanned by an electronic scanner and is encoded into the form of a string of digital data. It is possible to operate on this data to enlarge or reduce the size of the image, to apply colour corrections, and to vary the layout of pages to be printed. A set of printing members for four colour printing is then prepared directly from this data after it has been operated upon. Thus, the set of printing members is prepared without any initial visual inspection or proofing step. In view of the difficulty and expense of preparing such a set of printing members and the difficulty of setting up a conventional gravure colour press, it is desirable to be able to proof the digital data to ensure that it is satisfactory before undertaking the preparation of the set of printing members and undertaking the complete setting up of a conventional colour gravure printing press.